REPRODUCTION 65 



the environment as favor its continuance. Having in mind the necessi- 

 ties. of some relatively large animal, nutrition — the digestion and absorp- 

 tion of food — is best effected at an internal surface. The structures and 

 mechanisms immediately necessary for maintaining favorable relations 

 to the exterior — passively protective structures and motor-reaction 

 mechanisms — must lie at the external surface or in relation to it. Respir- 

 ation can be provided for at either an external or an internal surface. The 

 essential and minimum animal, then, is a two-layered thing. The outer 

 layer is protective, sensory, and may (as it actually does in coelenterates) 

 give rise directly to motor mechanisms. The inner layer is primarily 

 nutritive. It may (as in some coelenterates) share in producing motor 

 structures. Conceivably, and in view of the fact that all the functions of 

 a Uving thing are carried on by an organism so minute that it is organized 

 as a single cell, one layer might suffice. Given a small hollow animal, 

 its cavity opening to the outside and its single wall being one cell thick 

 throughout, nutrition might be attended to at the inner surface of the 

 layer while the externally superficial protoplasm might provide the neces- 

 sary contacts and adjustments with the exterior. But the principle 

 of speciahzation, "division of labor," early became manifest in the 

 evolution of animals. Two layers of cells, each layer speciaUzed for a 

 particular kind of function, work better than one. 



The gastrula is the animal in its bare essentials. The outer layer, 

 ectoderm, is potentially protective and nervous. It gives rise to the 

 essential part of the adult skin which produces so many important pro- 

 tective structures and to the whole nervous system, both peripheral and 

 central. The inner layer, endoderm, is nutritive. The cavity within it is 

 the primary digestive cavity or archenteron. It is significant that the 

 wall of the archenteron is derived from the vegetal hemisphere of the 

 blastula. Thus, appropriately, the greater quantity of yolk comes to 

 lie in the lining of the embryonic digestive cavity. In the vertebrates 

 the external aperture of the archenteron, the blastopore, never becomes 

 mouth and rarely becomes anus. The future motor mechanism, muscle, 

 is derived indirectly from the gastrula layers. 



The gastrula is strongly suggestive of the two-layer body plan of a 

 coelenterate. A simple coelenterate such as Hydra, two-layered through- 

 out, including even the tentacles, can be regarded as a somewhat elabor- 

 ated gastrula, the Hydra "mouth" corresponding to the blastopore 

 (Fig. 43). This resemblance, together with the fact that a gastrula 

 stage, modified in one way or another, occurs nearly universally in the 

 development of metazoan animals, gave rise to Ernst Haeckel's "gas- 

 traea" theory which proposed that gastrula-hke animals (essentially 

 coelenterates) must have been the ancestors of all Metazoa. According 

 to this theorv, the occurrence of the gastrula form in the ontogeny of a 



